King Charles is "in good spirits" amid his cancer battle, but nevertheless "hugely frustrated" by the pace of his recuperation, nephew Peter Phillips said Sunday, in the first detailed remarks on the monarch's condition by a member of the royal family.

Charles, 75, has been eager for a full return to his royal duties, Phillips - the son of Princess Anne, Charles' sister - told Sky News Australia.

"He's in good spirits," Phillips said. "[But] I think, ultimately, he's hugely frustrated. He's frustrated that he can't get on and do everything that he wants to be able to do."

"But he is very pragmatic," Phillips continued. "He understands that there's a period of time that he really needs to focus on himself."

After Charles underwent treatment for an enlarged prostate in January, Buckingham Palace announced in February that an unspecified form of cancer was found during the procedure.

Charles has been undergoing outpatient treatment while fulfilling his royal duties on a reduced schedule.

"He is always pushing his staff and everybody - his doctors and nurses - to be able to say 'actually can I do this, can I do that?'" Phillips said.

Phillips' comments came two days after Kate Middleton revealed that she is undergoing preventative cancer treatment, following a planned abdominal surgery in January.

In a statement issued Friday, Charles said that he is "so proud of Catherine for her courage in speaking as she did."